Mass spectrometers may benefit from collimated electron beams that allow for extended ion source geometries and optimal ionization efficiency per microampere of electron beam. Small electron guns that provide collimated electron beams with output currents ranging from picoamps to milliamps may be required to rapidly calibrate electron spectrometers. In such applications, the electron guns must be easily arranged in front of electron spectrometer apertures, and it may be helpful to have only a small number of electrodes to connect.
Ion sources may be used in mass spectrometers and in many other applications, including calibration of electron spectrometers for space plasma physics. The miniaturization of spectrometers may require a corresponding miniaturization in ion sources and ion sources' electron guns. Because of the increasing number of spectrometer suites in space plasma physics, a single instrument suite may include six or more electron spectrometers. Often, all of these spectrometers must be carefully calibrated to achieve the scientific goals. So, in addition to mass spectrometer applications, electron guns may be used to inject electron beams into the spectrometer apertures at different electron energies and angles of incidence. Smaller electron guns may be moved more easily than larger electron guns. Furthermore, if only a small number of electrodes are required to operate the electron gun, the risk of voltage breakdown of the electron gun may be reduced, for example, when calibrating for electrons exceeding several KeVs of kinetic energy.